Diesel engines for motor vehicles have good fuel economy and high durability. Unfortunately, the exhaust gas from diesel engines contains high levels of NOx and particulates. Both the United States and Europe have enacted regulations with strict limits on NOx and particulate emissions from diesel engines. The particulate and NOx limits as of 2004 for US trucks are 0.1 g/bhp-hr and 2 g/bhp-hr, respectively. The limits decrease to 0.01 g/bhp-hr and 0.2 g/bhp-hr in 2007.
Diesel particulate filters (DPFs) have been installed on buses and diesel automobiles for many years to remove the particulates from the exhaust stream. The filters may become plugged due to the buildup of particulates on the filter. The pressure drop through the filter may increase as the particulate levels on the filter increase. The plugged filter may have to be regenerated or replaced.
The particulates may comprise a mixture of lube oil solids, carbon particulates, and inorganic ash. The lube oil solids and carbon particulates can sometimes be removed from the DPF through combustion. The ignition temperature of the carbon particulates is normally about 600° C. Diesel exhaust temperatures are rarely that high. The exhaust gas temperature can be increased by retarding the timing but at the cost of fuel efficiency. Alternatively, the filter can be heated with an electric heater. Heating the electric heater requires energy, with an accompanying fuel efficiency penalty.
Johnson Matthey has described a system called “Continuously Regenerating Trap” (“CRT™”). A platinum-based diesel oxidation catalyst (DOC) is placed upstream of the DPF to oxidize NO in the exhaust stream to NO2. The NO2 in the exhaust stream may oxidize the carbon particles and lube oil solids on the DPF at lower temperatures than does the oxygen in the diesel exhaust gas. The CRT™ system is described, for example, in U.S. Pat. No. 4,902,487 to Cooper et al.
The platinum in the CRT™ DOC is expensive. Further, platinum catalysts may be poisoned by sulfur in the diesel fuel. Low sulfur diesel fuel is costly.
Catalysts have been dissolved or suspended in diesel fuel to lower the combustion temperature of the carbon particulates on the DPF. Use of fuel-borne catalysts requires additional components such as a tank for the fuel additive, a dosing system, and infrastructure to fill the additive tank. The fuel-borne additive can be expensive, particularly if the fuel-borne additive is a platinum group metal (PGM). Further, the fuel-borne additive can form particulate particles that may accumulate on the DPF, making it necessary to change the DPF.
Placing a catalyst on the DPF to lower the ignition temperature of the particles may be an attractive alternative to a fuel-borne catalyst. Hartwig (U.S. Pat. No. 4,510,265) describes a catalyst comprising a platinum group metal and silver vanadate. The catalyst of Homeier (U.S. Pat. No. 4,759,918) comprises platinum, palladium, or rhodium on a sulfur resistant support such as titania or zirconia. Dettling (U.S. Pat. No. 5,100,632) utilizes a catalyst that is a mixture of one or more platinum group metals and one or more alkaline earth oxides such as magnesium oxide. The catalysts of Harwig, Homeier, and Dettling al. comprise platinum group metals (PGMs). Platinum group metal (PGM) catalysts are expensive.
DPF's that contain vanadium catalysts to lower the combustion temperature of the carbon particulates have been described in U.S. Pat. No. 4,900,517 to DeGussa A. G. Other vanadium catalysts are discussed in U.S. Pat. No. 6,013,599, assigned to Redem.
Vanadium oxides are volatile and toxic. The high temperatures that are present in the DPF during combustion of the carbon particulates can vaporize the vanadium catalysts on the DPF, potentially leading to health problems in the general populace. There is a need for DPF catalysts that do not contain expensive PGMs or toxic vanadium compounds.
The surface area of many DPF catalysts is low. Adding a washcoat to the DPF to support the catalyst can increase the surface area of the catalyst by dispersing the catalyst on the washcoat. There is a need for a washcoat that can be placed on the support to increase the surface area of the supported DPF catalyst.